(a) Field of the Invention
The present invention relates to a derailleur drive chain structure for downhill event, and more particularly to one having a protruding portion provided on each outer chain plate to improve tensile strength of the chain.
(b) Description of the Prior Art
As illustrated in FIG. 9 of the accompanying drawings, a chain (6) of a bike when operates, a sleeve (62) of a chain plate (61) fully contacts a tooth root (72) of a chain tooth (71), and a first side (611) of the chain plate (61) (including an inner chain plate and an outer chain plate) that stays closest to the chain tooth (71) of a sprocket (7) when contacting the tooth root (72) of the chain tooth (71) must not contact the sprocket (7) to prevent the chain plate (61) from being lifted by the sprocket (7) or its chain tooth (71) because the lifted chain plate (61) will result in instantaneous disengagement of the chain (6) from the sprocket (7) or incomplete engagement between the chain (6) and the sprocket (7) to endanger the safety of the rider of the bike.
As for the multi-stage speed variable chain adapted to a bike for downhill event, safety concerns become more important due to the bike rides much faster. Accordingly, while requiring precise positioning of the speed variation, the chain must be provided with extremely tough tensile strength. The impact withstanding strength is also much greater than the general speed variable chain. Therefore several ways to increase the strength of the speed variable chain, including:                1. getting the chain thicker; or        2. enlarging the diameter of each pivoting end on both sides of the chain plate.        
Taking the 9-speed variable flywheel for example, as limited by 2.56 mm of the clearance between both flywheel plates, 1.76 mm of the thickness of the gear, and approximately 0.5 mm of the pitch of the smallest tooth root among the flywheel plates, the total height of the chain is limited to 6.6 mm. Furthermore, the outer edges of both pivoting ends on both sides of the chain plate (6) are symmetric to the center, and the chain plate (6) can only be thickened to a certain extent for being subject to those same conditions that constrain the total height of the chain. Whereas the total height of the chain is prevented from getting greater than 6.6 mm, the attempted increase of the diameter of each pivoting end on both sides of the chain plate is also limited because the diameter is prevented from getting greater than 4.25 mm; otherwise, each pivoting end will hit the bottom of the smallest flywheel plate. Therefore, it is very difficult to increase the strength of the chain.